A secondary battery for memory back-up is soldered by so-called reflow soldering in which a soldering cream is applied to a portion of a printed board where the battery should be placed, the battery is placed on the soldering cream, and the printed board is passed through a reflow furnace at a temperature of 200˜270° C. to solder the battery to the board.
Such battery (hereinafter sometimes referred to as a reflow battery) comprises heat-resistant electrodes, electrolyte, separator and the like because it is necessary for the reflow battery to be heat-resistant to the reflow temperature during reflow soldering (JP Laid-open Publication No. 2001-148242).
Although electronic equipment has been highly efficient and highly reliable, sufficient storage characteristics have not been obtained for a reflow battery used for memory back-up.
In the lithium secondary battery, a negative electrode and an electrolyte react during storage because the active material of the negative electrode contains active lithium. Many different electrolytes have been investigated for the lithium secondary battery. However, a desirable electrolyte has not been developed.